SOI technologies have been investigated extensively for a number of applications including rad-hard devices, high-performance high-speed IC's, and 3-D integrated devices. SOI formation may be done by several different wafer bonding techniques. These techniques employ various bonding methods such as annealing, mechanical pressing and annealing, or electric-field pressing and annealing. Various interlayer insulators such as thermal SiO.sub.2 and doped oxide have been used. Typical wafer bonding processes, based on the Van der Waals forces require extremely flat, particle free and defect free surfaces. After the two wafers are bonded together, various thinning methods may be used on the active SOI layer.
One bonding method employs a thin layer of metal, such as Ti or Al, between two nitride layers, on two wafers, which are then pressed together. A bonding by glass layer fusing technique bonds two glass layers with low softening/flow temperature. This technique, however, has been based on alkali lead borosilicate spin-on glass which can contaminate the wafer. A technique has been developed for direct bonding of Si wafers. The bonding process employs contacting of hydrogenated Si on SiO.sub.2 surfaces, which are flat with a high surface finish, and an anneal step above 900.degree. C. in an oxidizing ambient. In a process using Van der Waals forces and chemical bonding, two oxidized wafers are contacted after cleaning in an aqueous environment. As a result, the oxidized surfaces will contain SiOH groups on which additional water molecules are bonded by hydrogen bonding. The resultant Van der Waals forces are about 10.sup.6 N/M.sup.2 between two 100 mm wafers. The wafer pair is then annealed at 1050.degree.-1150.degree. C. This results in a chemical reaction at the interface. The Van der Waals forces are converted to much stronger chemical bonds. The soot bonding process employs the following steps: spraying Si-B-O soot generated by a flame hydrolysis reaction, positioning a support substrate, and sintering. The soot particles are sprayed against heated grooved substrates. The surface of the deposited layer becomes almost flat because of selective filling of grooves. The active substrate is placed against a holding substrate and heat treated.
Generally, conventional wafer bonding techniques require particle-free surfaces, surface flatness and polish, and most require high temperature anneal steps (e.g. 1000.degree.-1280.degree. C.).